Annual Occurrence Rates of Ionospheric Polar Cap Patches Observed Using Swarm

Abstract

Dense, fast‐moving regions of ionization called polar cap patches are known to occur in the high‐latitude F region ionosphere. Patches are widely believed to be caused by convection of dense, sunlit plasma into a dark and therefore low‐density polar cap ionosphere. This leads to the belief that patches are a winter phenomenon. Surprisingly, a long‐term analysis of 3 years of ionospheric measurements from the Swarm satellites shows that large density enhancements occur far more frequently in local summer than local winter in the Southern Hemisphere (SH). The reverse is true in the Northern Hemisphere (NH). Previously reported patch detections in the SH are reexamined. Detection algorithms using only a relative doubling test count very small density fluctuations in SH winter due to extremely low ambient densities found there, while much larger enhancements occurring in SH summer are missed due to especially high ambient densities. The same problem does not afflict results in the NH, where ambient densities are more stable year‐round due to the ionospheric annual asymmetry. Given this new analysis, the definition of a patch as a doubling of the ambient density is not suitable for the SH. We propose a test for patches linked to long‐term averaged solar flux activity, characterized by the 81 day centered mean F10.7 index. Importantly, the current patch formation theory is at least incomplete in that it does not predict the observed lack of patches in SH winter, or the many large enhancements seen in SH summer.

Climatology of GPS signal loss observed by Swarm satellites

“…Our result shown in Fig. 6 reveals that in both hemispheres the GPS signal losses have a higher occurrence during December solstice, supporting the conclusion of Noja et al (2013) and Chartier et al (2018) that the polar patches have higher occurrence at this season. The possible reason for explaining the different seasonal dependence of polar patches is that Spicher et al (2017) used a method with relative threshold for identifying polar patches (the enhancement must be at least twice that of the background density), while the detection approach used by Noja et al (2013) was based on a mixture of absolute and relative patch magnitudes (besides the relative enhancement, the absolute TEC enhancement must be larger than 4 TECU; 1 TECU = 10 16 electrons m −2 ).…”

“…However, the seasonal dependence of polar patches is still an open issue: some studies reported that the polar patches are mainly a local winter phenomenon in both hemispheres (e.g., Coley and Heelis, 1998;Kivanç and Heelis, 1998;Carlson, 2012;Spicher et al, 2017), while there are also studies found that polar patches have higher occurrence during December solstice months in both hemispheres (e.g., Noja et al, 2013;Chartier et al, 2018). Our result shown in Fig.…”

On the Annual Asymmetry of High‐Latitude Sporadic F

“…Given that sporadic F region enhancements are made up of photoionized plasma convected into the polar caps from the subauroral dayside (e.g., Carlson, ), the well‐known global annual asymmetry (e.g.,Mendillo et al, ; Rishbeth & Müller‐Wodarg , ) also plays an important role in driving the observed pattern of variability, as it controls plasma levels in the supply region. These factors combine in SAMI3 to reproduce the pattern of variability observed here and by Noja et al () and Chartier et al (), with Δ N (as characterized by the range of TEC values present in the polar cap) larger in January than in July in both hemispheres.…”

“…Notable observations that appear to support that theory include Coley and Heelis () and Spicher et al (), who counted more F region density spikes in local winter in each hemisphere. Chartier et al () explained that the apparent contradiction between Noja et al () and the others is the result of different counting metrics: Noja et al () tested for a proxy of Δ N (topside TEC enhancements >4 TECU) , whereas the typical approach has been to look for changes in Δ N / N , expressed perhaps most famously in Crowley's () test specifying that a patch must be greater than twice the background density. This choice of a different metric explains why Noja et al () got a different result than the others, but it does not explain why Δ N should have an annual variability with a marked minimum in July.…”

Hemispherical Shifted Symmetry in Polar Cap Patch Occurrence: A Survey of GPS TEC Maps From 2015–2018

“…More recent studies in which the in situ electron density measurements from the CHAMP and SWARM satellite systems have been used to compare the morphology and frequency of patch occurrence in the Northern and Southern Hemispheres include Noja et al () and Chartier et al (). The results reported in these two studies are distinctly different from those we obtain here in the present study; both conclude that patches in both hemispheres occur most frequently during the same months of the year, and not in each hemisphere's winter months.…”